A SURFACE-TENSION METHOD FOR DETERMINING BINDING CONSTANTS FOR CYCLODEXTRIN INCLUSION COMPLEXES OF IONIC SURFACTANTS

A new method has been developed for determining binding constants of complexes of cyclodextrins with surface-active compounds, including water-soluble ionic surfactants. The technique requires measuring the change in surface tension caused by addition of a cyclodextrin (CD) to aqueous solutions of the surfactant; the experimental results lead directly to inferred values of the thermodynamic activity of the surfactant. Surface tension results are reported for three different surfactants (sodium dodecyl sulfate (SDS), cetylpyridinium chloride (CPC), and cetyltrimethylammonium bromide (CTAB)) in the presence and in the absence of added beta-CD. Data for CPC have been obtained at surfactant concentrations below and above the critical micelle concentration. Correlations between surface tension and surfactant activity are expressed by the Szyszkowski equation, which subsumes the Langmuir adsorption model and the Gibbs equation. It is observed that the surface tension increases monotonically as beta-cyclodextrin is added to ionic surfactant solutions. At concentrations of CD well in excess of the surfactant concentration, the surface tension approaches that of pure water, indicating that neither the surfactant-CD complexes nor CD itself are surface active. Binding constants are inferred from a model that incorporates the parameters of the Szyszkowski equation and mass action constants relating to the formation of micelles from monomers of the surfactant and the counterion. Evidence is given that two molecules of CD can complex the C-16 hydrocarbon chain of the cetyl surfactants.